The ezo momonga, a subspecies of Siberian flying squirrel endemic to the island of Hokkaido in Japan, is often described as a “passive glider.” But Tokyo-based photographer and naturalist Tony Wu feels that’s a bit of a misnomer, even if technically correct. Just “no,” he says. “They are tiny F-15 jets driven by pilots on amphetamines.”
Known to the scientific community as Pteromys volans orii, the ezo momonga can execute aerobatics that seem astonishing for an animal without true wings, including lightning-fast twists, turns, flips, and steep dives. The show occurs mainly during the spring breeding season, when males battle each other for females, and females sometimes sneak away for liaisons with partners other than their mates.
Since the start of the pandemic, Wu has spent time each year on Hokkaido with his wife Emiko Miyazaki Wu, an avowed ezo momonga fan, trying to spy on the usually-nocturnal critters. Spring is optimal because the squirrels occasionally emerge from their nests during daylight hours in their quest to secure mates. They favor mixed-conifer forests that blanket the island’s mountains, and usually nest in tree cavities—either natural cracks or hollows, or holes left behind by woodpeckers. Nest trees can be identified by the halo of squirrel poop around the base, as well as snow spattered with pee let loose from on high, and debris from their snacks of buds, seeds, and leaves. Wu photographed this individual during a scuffle with another male, zooming between trees with the help of membranes, called patagia, that stretch along each side of the body between the squirrel’s front and rear legs, which can carry the animal on glides of up to 50 meters.
Ezo momonga are tiny, no more than 4.2 ounces in weight—less than a typical letter sent through the post—and 7 inches long, excluding tail, with large, liquid black eyes that have earned the subspecies comparisons to Pokemon. But don’t be fooled by their off-the-charts cuteness. Wu has also photographed male ezo momonga body-slamming their mating competitors off of trees. Once, he even saw a squirrel fleeing from another male execute an elaborate fake and escape. “The first [squirrel] went four to six meters, did a complete u-turn, and the other kept going in the wrong direction,” Wu says. Seeing something like that, “you just go, ‘Oh! How?!’”
The subspecies likely split from its Siberian flying squirrel cousins, Pteromys volans, during an interglacial period between 200,000 and 400,000 years ago, when seas fed by melting glaciers rose over the land bridges connecting them to the mainland. Pteromys volans ranges from Eastern Europe throughout Asia, and the International Union for Conservation of Nature considers it a species of least concern. Still, it’s thought to be generally declining, and is extinct in Lithuania and possibly Belarus. The squirrels are reluctant to cross open areas, perhaps due to vulnerability to predators and wind, and therefore need unfragmented forests to thrive. The Japan subspecies is common, but it still faces threats from logging and development, just as its relatives do in most places where they coexist with people.
On Hokkaido, Pteromys volans orii generally prefer the Sakhalin fir, Abies sachalinensis, a conifer that tends to develop large cracks and cavities as it ages, which the squirrels then fluff with shredded bark and dry moss to create their nests. That might sound plenty cozy, but during the island’s famously snowy winters the squirrels get even cozier by shacking up several to a nest to share body heat. Needless to say, depending on the mix between male and female, this arrangement usually falls apart come body-slam season, Wu says.
Sarah Gilman is a writer, illustrator, and editor who covers the environment, science, and place from rural Washington state. She's also a contributing editor at Hakai Magazine. Her work has appeared in The Atlantic, Audubon, Smithsonian, High Country News, National Geographic, and others.